After a cockfight atrocity in which 20 people were slain, an American mother died and her teenage sister is critically injured.

Melissa Silva, 16, and her sister Arleth Silva, 16, were on vacation in Michoacan, Mexico, when shooting broke out, according to family.

Melissa, a mother of four, was killed, and Arleth was shot several times.

During the brutal slaughter in the Las Tranjas district, seventeen men and three women were killed.

Arleth is in serious condition in a Mexican hospital. She was one of four people injured in the shooting.

A family member stated on a GoFundMe website that she was shot once in the lungs and twice in the legs.

According to prosecutors, the gunmen planned the attack and arrived in the region in a stolen vehicle owned by a snack food firm.

According to the publication, “the snack food company truck came, and six armed guys in camouflage attire got out,” according to a prosecutors’ statement.

“At the same time, a bus outside the building was deployed as a barricade,” it claimed, apparently to prevent victims from fleeing or phoning for help.

The incident occurred just after 10:30 p.m. Sunday, when a suspected gang of cartel members dressed in military uniforms stormed the crowded event.

Investigators discovered 15 automobiles that were reportedly owned by the victims. One of them wore a sticker with a criminal gang’s insignia on it.

Over 100 assault weapon ammunition were also located on the ground by cops.

Drug cartels and other gangs have been battling in the neighbourhood, according to prosecutors.

“There are suspicions that the attack was the result of a conflict between criminal gangs,” the federal Public Safety Department stated in a statement.

Mexico’s president, Andres Manuel Lopez Obrador, blamed the deaths on local criminal groups and promised a thorough investigation.

“It was a slaughter committed by one group against another,” he said during a press conference.

Mexican army and national guard forces were sent on Monday to apprehend those responsible.

Violence continues to rise

Last month, it was found that gang violence in Mexico is still on the increase, despite a violent new cartel battle.

The violence has even spread to renowned tourist destinations like Cancun and Playa del Carmen.

Two Canadians were murdered in January by a hitman who used a wristband to bypass security at the Hotel Xcaret resort south of Playa del Carmen.

In November, four Americans were injured in a violent firefight on a beach near the five-star Hyatt Ziva Riviera in Cancun.